1. Technical Field
The present exemplary embodiments relate to an MRI (magnetic resonance imaging) apparatus and a magnetic resonance imaging method which magnetically excites nuclear spins of an object with an RF (radio frequency) signal having the Larmor frequency and reconstruct an image based on an MR (magnetic resonance) signal generated due to the excitation, and more particularly, to a magnetic resonance imaging apparatus and a magnetic resonance imaging method, which performs a non-contrast-enhanced MRA (magnetic resonance angiography) obtaining a blood flow image without using a contrast medium.
2. Related Art
Magnetic resonance imaging is an imaging method which magnetically excites nuclear spins of an object set in a static magnetic field with an RF signal having the Larmor frequency and reconstructs an image based on an MR signal generated due to the excitation.
In the field of magnetic resonance imaging, as a method of obtaining an image of a blood flow, MRA is known. MRI that does not use a contrast medium is referred to as non-contrast-enhanced MRA. As non-contrast-enhanced MRA, an FBI (fresh blood imaging) method which performs in synchronism with an ECG (electrocardiogram) signal to capture a rapid blood flow ejected from the heart, thereby satisfactorily representing a blood vessel has been devised (for example, refer to Japanese Patent Application (Laid-Open) No. 2000-5144).
Such non-contrast-enhanced MRA using the FBI method creates a difference between image data acquired with different delay times of ECG synchronization so that an MRA image in which an artery and a vein are distinguished from each other is obtained. In addition, in the FBI method, a flow-spoiled FBI method in which an artery signal is suppressed during systole by applying a spoiler pulse has been devised. That is, according to the flow-spoiled FBI method, the difference of artery signals during diastole and systole of the cardiac muscle is used for imaging.
Further, in the FBI method, in order to extract a blood flow of low flow velocity, a flow-dephasing method in which a gradient pulse (Gspoil) is applied in a RO (readout) direction, and a dephase pulse or refocusing pulse is applied to a gradient magnetic field pulse has been designed (for example, refer to JP-A-2003-135430). According to the flow-dephasing method, due to the dephase pulse or the refocusing pulse, it is possible to increase the relative signal difference between a signal value from the blood flow of high velocity and a signal value from the blood flow of low velocity. Therefore, it is possible to clearly distinguish the artery and the vein from each other on the basis of the relative signal difference.
That is, in order to distinguish the artery and the vein, it is important to increase the difference between signals during diastole with respect to signals during systole. In order to increase the difference between signals in diastole and systole, it is needed to make intensity of the signal from the blood flow in systole small. However, especially in case of MRA for a lower limb, since flow velocities of both venous blood and arterial blood are slow, it is difficult to distinguish the artery and the vein due to decrease of signal difference between diastole and systole. Accordingly, a gradient pulse having a proper intensity in the RO direction is set, and the blood flow signal from the artery in systole is suppressed by the set gradient pulse. Thus, it is possible to increase a difference of signals from the artery between diastole and systole. In this state, the blood flow signal in diastole is collected. Subtraction processing and/or MIP (maximum intensity projection) processing are performed on the blood flow signals collected in diastole, and only the artery is represented.
Further ECG-prep as a related technology used with FBI method has been devised to measure an appropriate delay time for ECG synchronization (for example, refer to U.S. Pat. No. 6,144,201). The ECG-prep performs an ECG-prep scan which is a preparation scan to decide an appropriate delay time for ECG synchronization previous to an FBI scan for imaging, and subsequently performs the FBI scan with the ECG delay time decided by the ECG-prep scan. The ECG-prep scan obtains plural single shot images at mutually different time phases by acquiring data at gradually changed different delay time from an R wave of an ECG signal. By selecting an image where a blood vessel is depicted appropriately from among the plural images obtained from the ECG-prep scan, an ECG delay time for the FBI scan can be determined. This allows a high-velocity blood flow to be depicted at a time phase corresponding to a lower flow velocity.
In the conventional Flow-Spoiled FBI method, an FASE (fast advanced spin echo) sequence is used as an imaging sequence and a gradient pulse is applied in an RO direction to suppress a blood-flow signal from an artery systole. In a technology, called Flow-Adjusted FBI, a gradient pulse is applied not only in an RO direction but in a PE (phase encode) direction. A gradient pulse application achieves good arteriovenous separation.
However, when a spoiler intensity of a gradient pulse is increased in order to improve arteriovenous separation performance, it is necessary to extend ETS (echo train spacing). As a result, there are problems in that time resolution decreases with an increase in ETS and arteriovenous separation of a high-velocity blood vessel becomes difficult.
In the conventional Flow-dephasing method, since a dephasing pulse is applied to a gradient magnetic field in an RO direction in a scan under the FASE method, a depiction performance of blood vessels depends on a blood vessel direction and a depiction performance accordingly has a limitation.